1. Field of the Invention
The present invention relates generally to optical systems, and, in particular, systems that use ultraviolet radiation for materials processing. In its preferred embodiment, the present invention is used for the well-controlled treatment of vapors, such as in the vapor deposition of thin films. Potential applications exist in the fabrication of integrated circuits, optical elements, optoelectronic devices, and other such materials processes.
2. Description of the Related Art
In one aspect, the invention relates generally to the treatment of dispersed photoabsorbing media, such as gases, with ultraviolet radiation. An equipment geometry for this purpose, in fluid treatment, utilizes a flow-through geometry, wherein the media to be processed passes through a processing tube constructed of ultraviolet-transmitting materialxe2x80x94such as fused silicaxe2x80x94and wherein the tube is surrounded with one or several ultraviolet lamps, thereby creating a high radiative flux within the photoabsorbing media. The coupling efficiency of the ultraviolet radiation to the media may then be increased, by placing this coaxial arrangement within a reflective cavity. This latter reflective cavity becomes increasingly necessary as the extinction distance of the ultraviolet within the media becomes much greater than the relevant physical dimension of the apparatus, and the ultraviolet radiation must make many passes through the media before it is appreciably absorbed.
When the extinction distance is orders of magnitude longer than the relevant cavity dimension, the coupling efficiency of this ultraviolet source becomes inherently limited by parasitic losses within the reflective cavity, which xe2x80x9cstealxe2x80x9d the ultraviolet radiation away before it can be absorbed by the dispersed media. Similarly to a laser source, such losses can be largely attributed to a combination of the mirror quality, diffraction losses, and the propagation of the ultraviolet along optical paths which xe2x80x9cwalk offxe2x80x9d the cavity mirrors in so-called xe2x80x9cend-lossesxe2x80x9d. Unlike a laser, however, retention within the reflective cavity of these lamp sources is not greatly increased through the establishment of high-retention lasing modes that dominate the photoemission process. In addition, as clearance restrictions require the aforementioned processing tube to be increasingly short in lengthxe2x80x94or of increasingly smaller aspect ratioxe2x80x94the end-losses of the commensurately shorter reflective cavity become a serious limitation to the coupling efficiency of the processing apparatus. Conversely, ultraviolet radiation that is allowed to leak from the reflective cavity may adversely interact with other parts of the process. These sort of requirements have been partially dealt with in past fusion research, at least when using longer optical wavelengths, by implementing a laser cavity. In this approach, the photoabsorbing media is passed through the laser cavity itself. However, this approach becomes far more difficult for other, similarly configured, materials processes that require ultraviolet radiation; in part, because these latter processes would typically require much more economical solutions than those afforded fusion research, whereas, capital and maintenance costs for a high-power ultraviolet laser are likely to be prohibitively high.
The problems encountered with irradiating low absorption cross-section dispersed media become increasingly acute with lower pressure processes, wherein the dispersed media would typically be some gas or vapor which is rarified to a degree consistent with the level of vacuum. In these latter vacuum processes, one encounters situations wherein the absorbing constituent may have a vapor pressure of only 10xe2x88x926 atmospheres, with extinction distances in the range of 102 to 103 meters. At the same time, these vacuum processes will frequently involve one or several critical material surfaces that interact with the process quite differently when irradiated with the ultraviolet radiation. These same critical surfaces will typically be modified during the process, so that the result of irradiating these surfaces will change, as well. For instance, a thin film forming on one of these material surfaces can dramatically alter the absorption, scatter, or reflection of the ultraviolet radiation as its thickness increases. In ultraviolet-enhanced physical vapor deposition (PVD) processes, including reactive processes utilizing PVD sources, these issues have not been adequately addressed.
A prevalent PVD means in industry for the deposition of high quality thin films is through the utilization of sputtering techniques. The term xe2x80x9csputteringxe2x80x9d refers to a group of mechanisms by which material is ejected from a solid, or sometimes a liquid, target surface into a vapor form; this latter effect being due, at least in part, in either physical or chemical sputtering, to the kinetic energy transferred to the target atoms or molecules by bombarding particles. These mechanisms are utilized in sputter deposition processes categorized generally as laser sputtering, ion beam sputtering, glow discharge (or diode) sputtering, and magnetron glow discharge sputtering. The present invention, in its preferred embodiment, concerns primarily plasma sputtering, and, in particular, magnetron plasma sputtering. The magnetic confinement of the sputtering plasma in the magnetron sputtering process allows for a far greater range of mean free paths than the earlier, capacitively coupled diode plasma sputtering process. Its high deposition rate, combined with its versatility in depositing a wide range of materials under a great range of conditions, has made magnetron plasma sputtering a preferred thin film deposition technique for many industrial applications.
Yet, there are several aspects of plasma sputtering which are seen as significant barriers in utilizing the technique for future industrial applications. Most commercially available plasma sputter sources provide a small proportion of ionized species to the depositing film ( less than 5%). Most of the energy supplied for non-equilibrium growth is supplied by the thermal velocities of the depositing species. The thermal distribution of these velocities is necessarily broad, allowing little control over specific growth processes at the film growth interface. Because the energy supplied by the depositing species is kinetic, it is often difficult to provide high energies to the growth interface with out simultaneously causing subsurface damage, due to the recoil and implantation of the bombarding atoms.
Several modifications have been devised to render greater control over plasma sputtering processes wherein, as in the present invention, excited state and ion populations in the gas/vapor phase are increased and manipulated by means external to the sputtering plasma. This is most commonly accomplished by injecting electrons into the sputtering plasma to increase the plasma density and ion population, while simultaneously allowing a decrease of the target voltage. A resulting benefit is the ability to introduce a high proportion of relatively low energy ions to either etch or deposit on the substrate. This method has been made popular in the well-established triode and tetrode sputtering configurations, wherein electrons are usually supplied by a thermionic filament. This latter art has been found to work well for the deposition of metals, but is not compatible with reactive processes where electron emitting surfaces are prone to modification.
In recent years, plasma sputtering processes have also been developed that increase ionization through the utilization of secondary coils or antennas for RF or microwave excitation of the plasma. This latter prior art has also been found useful in the deposition of metals. However, difficulties arise, in that resonance conditions are effected by the inevitable modification of the process chamber surfaces during deposition, especially when depositing insulating or semi-insulating materials; also, these latter developments offer little resolution of the plasma species to be ionized.
The use of sources of UV/optical energy in conjunction with sputtering plasmas is relatively limited compared to the prior art concerning electron sources. In various instances, plasma sputtering experiments have been conducted utilizing the geometry set forth in U.S. Pat. No. 4,664,769 issued May 12, 1987 by Cuomo et al. This patent teaches a method wherein a UV source is directed onto a sputtering target during the magnetron sputtering process. The UV wavelength used is of an energy of or exceeding the photoelectric threshold of the target material, thereby causing the target to emit photoelectrons into the sputtering plasma. This photoelectric addition of electrons is found to increase plasma density, lower the cathode voltage required to sustain a discharge, as well as to increase the ion flux to the substrate, enabling modification of the film properties. As this work focuses on the irradiation of the sputtering target, its operation is contradictory to the goals of the present invention.
The use of UV/optical sources with magnetron sputtering plasmas in later work has consisted of efforts wherein a UV source, usually a laser, is directed upon the substrate being processed. These experiments are conducted in order to promote and study various surface reactions and solid phase transformations at the substrate surface, sometimes with a reactive gas injected at the substrate. As such, these accounts deal with UV interactions with the substrate surface and do not anticipate the present invention.
The use of UV/optical radiation sources in combination with processing plasmas has consisted mostly of the research conducted in relatively higher pressure photo-enhanced and plasma-enhanced chemical vapor deposition (CVD) processes. In the relevant accounts, ultraviolet radiation, usually from a laser, irradiates the substrate upon which the thin film is being deposited. This work in CVD was originated by Hargis, Gee, et al, and reported in the publications, xe2x80x9cLaser-plasma interactions for the deposition and etching of thin-film materialsxe2x80x9d, wherein is described the mechanism by which laser-produced UV activates the top monolayers which are plasma-deposited on the substrate.
This initial work utilizing both plasmas and UV radiation sources in CVD has continued. Researchers have since found that the plasmas used for plasma-enhanced CVD and plasma-enhanced chemical etching may be simultaneously or separately used as a photochemical UV source. The interaction of UV with these plasma-enhanced CVD and chemical etch processes has been found to take place primarily in surface modifications, such as in photo-activation of heterogeneous surface reactions at the substrate surface; because of this, these process geometries must incorporate means for illuminating the substrate surface which is being modified. Any photo-activated gas-phase reactions which might, in addition to the surface interactions, occur in these CVD and chemical etching plasmas would be essentially non-existent in the low-pressure, higher power density sputtering plasmas; nor are such gas phase reactions a necessary element of the present invention.
While the use of UV sources is a promising route for enhancing and controlling film growth and etching processes in plasma processing, the aforementioned prior art has had little impact on sputtering deposition/etching applications. Reasons for this are viewed, in the present invention, in light of the highly non-equilibrium thermodynamic mechanisms inherent in plasma sputtering technology. While the plasma sputter source provides a reliable means for depositing many materials under a wide range of conditions, consistently achieving a specific resultant film structure and composition, within relatively tight tolerances, remains a formidable challenge. Introducing additional energy sources to the sputtering plasma further complicates issues of stability and repeatability.
The prior art invariably utilizes process-altered surfaces which receive UV energy, namely the sputtering target or the substrate; but, in addition, chamber walls and fixturing. Any solid surfaces which might potentially receive UV radiation must act as a transmitting, reflecting, absorbing, or scattering surface. Because these process surfaces tend to be altered during the deposition process, the interaction of the UV source with the deposition process is also altered. As the reflectivity, scattering, and absorption occurring at these surfaces changes with process time, plasma-related mechanisms occurring throughout the process volume, such as secondary electron emission, gas/vapor photo-excitation, radiant heating, and photon-assisted sputtering, can all be dramatically altered. Hence, UV radiation incident on a growing film, or on the sputtering target, can interact with the deposition process in an unstable fashion.
Another problem arises in the use, in previous experiments, of UV (and other) sources which interact, in particular, with magnetron plasmas in a highly asymmetric geometry. The relative stability and repeatability of a conventional magnetron plasma source is due, in great part, to the maintenance of a symmetrically uniform magnetic and electric field for containment of the magnetron plasma. For example, a rectangular magnetron is more prone to arcing, in D.C. reactive sputtering, than a circular magnetron, due to its lower symmetry. As such, any UV source which is made to interact with a magnetron plasma should preserve the magnetron""s symmetry, if stable, repeatable performance is to be maintained.
The terms xe2x80x9cplasmaxe2x80x9d and xe2x80x9cdischargexe2x80x9d both refer herein to the general sense of an electrically or electromagnetically sustained, photo-emitting, gas/vapor discharge, wherein quasineutrality of the gas/vapor may not necessarily exist. While the term xe2x80x9cplasmaxe2x80x9d has been used more restrictively, and certainly more inclusively, than in the definition offered herein, the latter definition is consistent with current-day usage in the semiconductor industry, vapor deposition sciences, and other areas where the present invention might find application. The two terms are utilized differently in the present disclosure as a means of clearly differentiating between the sputtering xe2x80x9cplasmaxe2x80x9d of the preferred embodiment, and the photoemitting xe2x80x9cdischargexe2x80x9d of the disclosed ultraviolet lamp source.
In disclosing the present invention, the terms xe2x80x9ccavityxe2x80x9d, xe2x80x9creflective cavityxe2x80x9d, and xe2x80x9coptical cavityxe2x80x9d, will all refer to the common and general sense of a predetermined structure for confining propagation of optical radiation between reflective surfaces. Also, the use of the term xe2x80x9cvaporxe2x80x9d will herein refer to any gaseous or vapor-like substance, as distinguished from a solid or liquid. This includes atoms, molecules, ions, clusters, and other such dispersed substances which may traverse the process space of a processing chamber.
The previously cited problems, as recognized by the present invention, are addressed herein, in part, through the development of a novel ultraviolet processing apparatus that provides both highly efficient coupling and well-controlled interactions with dispersed photoabsorbing media, such a vapors. According to one aspect of the present invention, a narrow-band UV source has been developed which represents a significant departure from previous incoherent or coherent light sources used in materials processing. The UV source disclosed herein utilizes an optical cavity which allows unusually high retention without requiring the establishment of optical gain. Unlike a conventional laser, light is not coupled through a partially reflective mirror to a medium external to the cavity, but instead, is coupled directly to dispersed photoabsorbing process media present in the central process space within the optical cavity. Since the process media is, in the preferred embodiment, a vapor with a necessarily low photo-absorption cross section, the xe2x80x9cactivexe2x80x9d losses in the cavity can be very low, allowing the cavity radiation density to increase proportionally, thereby increasing total absorption. The radiation density is also increased by virtue of the large effective numerical aperture achieved, as the symmetric cavity focuses the optical energy to a relatively small process space at the center of the cavity. As a quasi-monochromatic source, the reflective cavity is designed to be capable of higher efficiency and higher average radiation density than available ultraviolet-emitting lasers. Compared to previous UV lamps, the UV source developed herein can maintain relatively high overall efficiency in the both the generation of high radiation densities, and the utilization of the generated UV photons for a selected photo-absorption process. This efficiency is enabled through a configuration that delimits the process volume, in part, through the use of thin film optical interference means.
In accordance with the illustrated preferred embodiment, the present invention also provides a sputtering apparatus and method for the deposition of material on a workpiece, using intense ultraviolet (UV) radiation, including vacuum ultraviolet (VUV; xcex less than 200 nm) radiation, to irradiate vapor constituents of a sputtering process. In particular, the sputtering apparatus disclosed herein allows a very high degree of optical coupling, as well as a high symmetry and a high selectibility of optical coupling, between a high power, UV-producing lamp discharge and the sputtering plasma.
In its first preferred embodiment the invention includes a central circular magnetron electrode for sustaining the sputtering plasma, a peripheral optical aperture located above the electrode, a separate concentric volume containing means for generating high power, narrow-band UV emission, a concentric reflective cavity, means for positioning a workpiece to receive depositing species, and optical interference and collimation means for both spatial and spectral control of the UV emission within the sputtering plasma. The sputter source described herein utilizes the aforementioned UV processing apparatus, which confines UV radiation to propagate within a planar process space above the sputtering target (xe2x80x9cabove the targetxe2x80x9d will, throughout this text, refer to the side of the target exposed to vacuum). The energy and radiation density of the UV radiation produced by the disclosed UV apparatus is sufficient to ionize a vapor or gas constituent of the sputtering plasma.
Whereas the use of UV radiation offers the potential for highly resolved interactions with specific plasma species, prior plasma sputtering art utilizing UV has not been developed significantly as a production method. Prior art plasma sputtering experiments utilizing ultraviolet energy for altering plasma sputtering processes have invariably required direct illumination of a processing surface, namely, the target or workpiece; many of these interactions are found in the present invention to be inherently unstable and difficult to control in a sputtering environment. These latter UV-surface interactions also tend to dominate the photo-absorption process, especially at the low pressures used in sputtering processes, thereby severely limiting the proportion of the UV radiation actually absorbed through UV-vapor interactions. In the present invention, through the efficient containment of UV spectral emissions, effective collimation of the UV, and a symmetric distribution of UV about the sputtering plasma""s major axis of symmetry, a high density of UV radiation is available for stable and repeatable photo-excitation and/or photo-ionization of the gas/vapor-phase plasma species; at the same time, the interactions of this UV energy with the target, workpiece, or other process-altered surfaces, are profoundly reduced. Another related advantage of the UV source of the present invention, in its capacity as a UV-assisted processing apparatus, is the possibility of reducing UV-induced damage at the workpiece being processed.
The present invention provides a new and versatile method for fabricating thin film structures by enacting a greater degree of control over both the sputtering plasma parameters, and the nature of plasma-emitted particles, than conventional plasma sputtering processes. One advantage of the present invention is that it introduces a means for increasing ion densities in a repeatable, low-cost method. The present invention, in its preferred embodiment, offers distinct advantages over the prior art as a method for producing high ion densities in or above the sputtering plasma. This increase in ion density is achieved by using well-resolved, photo-ionizing UV-plasma interactions, in a configuration which substantially reduces the interaction of ionizing radiation with any process-altered surface. A resulting advantage is the introduction of a both repeatable and highly tailorable process for increasing ion densities in and/or above the sputtering plasma.
Another advantage of the present invention is the increased control allowed in achieving specific compositional or structural characteristics in the thin (or thick) film structures formed. The selective and reproducible ionization of a specific species in, or above, the sputter plasma allows control over the kinetic and chemical energies of those specific atomic or molecular species at the growth front of the film. The ability to control ion fluxes, chemical energy, and kinetic energies in vapor depostion are powerful tools in controlling the microstructure and crystallinity of a deposited film. Because of the thermodynamic non-equilibrium nature of the sputtering process, it may produce material phases and phase combinations not possible in more thermodynamically equilibrium processes, such as evaporation or the earlier diode sputtering. This is due to the energies supplied by the depositing atoms, and their reactions, at the growth interface, being representative of temperatures far exceeding that of the bulk temperature of the workpiece. Hence greater latitude in controlling these energies via mean free path (i.e., pressure) and ionization greatly increases the latitude available in achieving a particular film structure. At the lower sputtering pressures possible with higher ion densities, the resultant films deposited will also tend to contain less contamination by the sputtering gas (e.g., argon). This will, in turn, minimize structural defects induced by these gases, enhancing the use of sputtering as an epitaxial technique.
Higher ion fluxes in sputtering also greatly enable the effort to perform so-called metallic-mode reactive sputtering, wherein the target remains metallic during deposition and the reacted product is formed at the workpiece, as chemical activity of the metal atoms at the workpiece is greatly enhanced. This latter method is important for high-rate sputtering of dielectrics, as well as providing a promising technique in sputter epitaxy. Other objects related to the present invention follow.
An object of the present invention is to provide a self-contained means of irradiating dispersed photoabsorbing media.
Another object of the present invention is to provide a means for increasing the density of ionized and excited species in a vapor deposition process.
A further object of the invention is to provide a means of selectively activating desired transitions in a sputtering plasma, and thereby enacting a change in the properties of the deposited film.
Another object of the invention is to provide a means of irradiating a sputtering plasma and/or adjecent vapor with a high density of UV energy, in a manner which isolates the volume and nature of UV interaction.
Another object of the invention is to provide ionizing UV energy to plasma species without producing UV-induced damage at an adjacent workpiece.
Another object of the invention is to provide a novel, plasma-sustained ion source for the development of sputter-assisted processes.